A number of clear and pigmented coating compositions are utilized in various coatings, such as, for example, basecoats and clearcoats used in automotive refinish coatings, which are generally solvent based.
In repairing damage, such as dents to auto bodies, the original coating in and around the damaged area is typically sanded or ground out by mechanical means. Some times the original coating is stripped off from a portion or off the entire auto body to expose the bare metal underneath. After repairing the damage, the repaired surface is coated, preferably with low VOC coating compositions, typically in portable or permanent low cost painting enclosures vented to atmosphere to remove the organic solvents from the freshly applied paint coatings in a safe manner from the standpoint of operator health and explosion hazard. Typically, the drying and curing of the freshly applied paint takes place within these enclosures. Furthermore, the foregoing drying and curing steps take place within the enclosure to also prevent the wet paint from collecting dirt in the air or other contaminants.
As these paint enclosures take up significant floor space of typical small auto body paint repair shops, these shops prefer to dry and cure these paints as fast as possible. More expensive enclosures are frequently provided with heat sources, such as conventional heat lamps located inside the enclosure to cure the freshly applied paint at accelerated rates. Therefore, to provide more cost effective utilization of shop floor space and to minimize fire hazards resulting from wet coatings from solvent based coating compositions, there exists a continuing need for fast curing coating formulations which cure under ambient conditions while still providing outstanding performance characteristics particularly chip resistance, mar-resistance, durability and appearance.
In the past, several approaches have been used to improve the productivity of isocyanate crosslinked coatings. One approach was based on using higher Tg acrylic polymers (U.S. Pat. Nos. 5,279,862 and 5,314,953) and another on the use of reactive oligomers (U.S. Pat. No. 6,221,494 B1). Due to the high Mw and high Tg of such acrylic polymers, the fast dry was achievable, but the film vitrified and the faster cure was not achievable. The viscosity of these types of acrylic polymers was also comparatively high and thus that approach resulted in higher VOC. The reactive oligomer approach improved the appearance and the rate of cure of the coating at lower VOC, however these oligomers are difficult to make at high enough Tg needed to reduce the dry time. Moreover, the higher the amount of these oligomers in the coating compositions, the lower was the hardness of the resultant coatings.
Thus, a continuing need still exists for a coating composition that cures under ambient conditions, more particularly those compositions having low VOC.